Most information systems, such as personal computers and automotive controlling computers, rely upon a copper wire based network to carry signals to remote components. In many ways, this has proven to be an impediment to the speed of the system. At the center of a computer, for instance, a Pentium 4 central processor might operate at 2.4 GHz, but data travels on a central bus on a circuit board at a mere 400 MHz, while output devices might only receive their instructions at 133 MHz.
One approach is to push the wire to its theoretical limits. Physical dimensions, such as length and diameter, and the parasitic resistance, capacitance, and impedance resulting from the dimensions, however, define a wire. At low frequencies or bit rates, the series resistance and shunt capacitance of the wire (or circuit board trace) dominate its behavior. The rise or fall time limits the data rate. As the designer chooses to push the frequency higher, the wire's own impedance becomes the dominant factor, acting much as transmission lines to attenuate and reflect signals based upon the characteristic impedance of the lines. The fastest processor ends up waiting for the wires.
In high performance communication systems, photons have supplanted electrons as messengers. Photons travel on fiber optic waveguides from place to place at the speed of light. The photon frees the processor from its copper shackles. The market is just starting to see the several component-based optical networks capable of transmission rates of 2.5 Gb/sec. Chip-to-chip fiber optic connections could boost the output of a processor by a factor of a thousand.
One natural application for such a fiber optic network is military fighter aircraft. For the last century, the military has sought to integrate the latest technologies into the field of battle as a means of multiplying its strength. A modern fighter aircraft is effective because of the amazing array of computers, sensors, triggers, and displays onboard. The fighter is not only an airplane but it is also a combatant's instrument. In that role, it must defend itself, reconnoiter the theater of operations, track targets, jam enemy sensors, and fire its weapons. Because the weapons, themselves, are “smart,” the aircraft often must be competent to converse with the weapons in order to charge them with their mission before and during flight.
Not only is the aircraft an array of computers, but also among the array of computers, there exists, on the airframe, a great variety of computers. Some threat computers receive analog input—digitizing would slow reaction to the threat. Some engine sensors send analog information only. An optical network ideally should be able to handle both in order to be fully effective in optimizing the communication within the airframe.
Analog systems differ from digital systems. When transmitted over an optic fiber, analog and digital systems differ greatly in magnitude. All network systems have ambient noise. In an analog system, the approach to fidelity over ambient noise is to “shout” over it. Where an analog system has an anticipated range of values, the maximum value is set at a gain value representing the near maximum capacity of the fiber. The minimum value is designated as the first value reliably discernable over the noise. That value then becomes the bias for the analog system allowing fidelity in all information transmitted.
Digital systems have a slightly different approach to noise. Using the same ambient level as a threshold, only the “ones” have to exceed the ambient. The “zeros” can be buried in the noise without ill effect. In some systems where the “zeros” must emerge from the noise for the sake of timing, values for “zero” and “one” are set such that both exceed the noise but still do not approach magnitude of the analog signal. These, too, are “whispers” compared to the analog “shout.”
Systems within a fighter airframe are located at discrete places that allow optimal performance of the system. For instance, a wing might hold a variety of sensors: feedback loops for aileron and flap positions, anti-stall sensors, threat assessment antennae, fuel state sensors, and weapons state nodes. The information from each of the sensors must ultimately get to distinct systems on the airframe for processing.
One means of getting the information on the system has been to assign it to discrete optical “channels” according to the intended destination. By convention, the military has decided to designate 32 such channels for both analog and digital signals. The channels are partitioned to each of two common wavelength bands: 1.55 μm for analog signals and 1.3 μm for digital signals. Each of the channels has a rate capacity of approximately 2.5-40 Gb/second. Information senders are assigned to wavelengths according to the receiving system and according to the nature of the output, i.e. digital or analog.
Wavelength division multiplexing (WDM) is the process of carrying light of multiple wavelengths within the fiber. The fiber will carry the several channels without interference. WDM combines multiple optical signals so that they can be amplified as a group and transported over a single fiber to increase capacity. At the receiving end of the fiber, the information carried on the several channels is taken off of the fiber and separated into its several channels and recombined based upon destination.
Such a system presumes the existence of a router capable of sensing the wavelength of a channel carrier (arbitrarily λ1) from a WDM transmitter at a source and capable of placing the information from the designated channel onto the correct optic trunk to reach the intended WDM receiver destination in the system. Optical channels are separated and recombined most readily through the phenomena known as refraction and diffraction. Refraction is the deflection from a straight path undergone by a light ray or energy wave in passing obliquely from one medium (such as air) into another (such as glass) in which its velocity is different. Diffraction is a modification that light undergoes in passing by the edges of opaque bodies or through narrow slits or in being reflected from ruled surfaces and in which the rays appear to be deflected and to produce fringes of parallel light and dark or colored bands. In either regard, the light waves are bent according to their wavelength.
Many WDM multiplexers and de-multiplexers in current use, such as arrayed waveguide gratings (AWGs), are complex to fabricate, bulky in size, and relatively costly. The AWG consists of a number of arrayed channel waveguides that act together like a diffraction grating in a spectrometer. The grating offers high wavelength resolution, thus attaining narrow wavelength channel spacings such as 0.8 nm. Other de-multiplexers include traditional dispersive devices, such as diffraction gratings and prisms. While being much simpler and less expensive than AWGs, these devices typically have an angular dispersion less than one degree per nm, which prevents them from being sufficiently compact for most applications. Regardless of the configuration, because of their dependence on geometry, the routers are very sensitive to temperature and generally to shock, thus not well-suited to a military environment.
Superprisms, a much more highly dispersive photonic crystal counterpart to the array waveguide (AWG), have been used to map different wavelengths onto different propagation paths. Superprisms are simpler and much smaller than AWGs, and have very low cross-talk. They are a special type of photonic crystal de-multiplexing structure that provides angular separation by wavelength that is up to 100 times the angular separation of conventional dispersive media.
Photonic crystals are optical materials with an intricate three-dimensional structure that manipulates light in unusual ways thanks to multiple Bragg diffraction in specific directions. The structure has the length scale of the order of the wavelength of light. An example of a photonic crystal is the gem opal, which consists of a regular array of tiny silicate spheres, ordered like the atoms in a crystal lattice, but on a scale a thousand times larger. If the structure has a large enough variation in refractive index for a periodic array of holes or columns in specific directions relative to the symmetry of a crystal lattice, a “photonic bandgap” occurs. Under these special circumstances, Bragg diffraction prevents a certain range of wavelengths from propagating in selected directions inside the crystal. By designing the bandgap appropriately, i.e. by engineering the lattice spacing of the photonic material to be either highly dispersive or to blocks wavelengths (except those wavelengths passed by controlled defects, the resulting crystal will function as a superprism with dispersive properties for a superprism being many times greater than that of optical glass prisms. Such superprisms can be created by the same lithographic technologies that are currently employed for constructing integrated electronic circuitry. Alternately, controlled defects introduced into photonic crystal lattices can lead to wavelength selective resonator filters within the photonic band gap.
Currently WDM components occupy a volume or footprint that is too large for emerging military platforms. In addition, the WDM components will only support static network topologies, not allowing for new functions or services. Photonic crystal resonator filter components designed for digital power levels are expected to destructively overheat under the greater power outlay of analog systems. Analog systems require higher signal to noise ratios than is required to discern between the power levels assigned to “zeros” and “ones.”
What is needed then, is a WDM router on a chip-scale that overcomes problems with WDM components known in the art.